1. Technical Field of the Invention
The present invention relates to a cellular telecommunication network, and more particularly, to a backup to the network's digital control channel.
2. Description of Related Art
In modern cellular telecommunication networks, the networks are divided into cells which service a defined area for mobile stations. Each cell contains a base station which transmits and receives voice and control information to and from mobile stations which are located within the cell's coverage area. Each of the cells employs a separate control channel for relaying control information to mobile stations. Currently, cellular telecommunications networks utilize analog control channels for the passing of control information.
Analog technology, however, limits the total number of channels available within the mobile frequency spectrum to 832, approximately 21 of which may be used as control channels. Analog technology also limits the allocation of a subscriber communications or control channel to a single analog radio channel. Digital technology has been introduced to solve many of the limitations and problems associated with analog technology, therefore, the cellular telecommunication industry (hereinafter referred to as the cellular industry) is rapidly moving toward finalizing a standard for a Digital Control Channel (DCCH) which has total frequency agility over the entire mobile telephone frequency spectrum.
In digital cellular networks, a plurality of subscriber voice channels or control channels can be assigned to each radio frequency through Time Division Multiple Access (TDMA) technology. In TDMA technology, each channel which is broadcasting at a particular frequency is divided into a plurality of time slots. Subscriber communications or control channel signals are converted to digital format and divided into short communications bursts. Each burst is tagged with an identifier, assigned a time slot, and is broadcast in an interleaved fashion with other bursts on the same frequency. At the receiving end, the identifiers are used to reconstruct the entire communication from the individual bursts.
The current version of the cellular industry standard for a Digital Control Channel (DCCH) is described in Project No. 3011-2 of the EIA/TIA Interim Standard IS-54-C, "Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard", dated Apr. 8, 1994, which is hereby incorporated by reference herein. As noted above, the DCCH provides total frequency agility over the entire mobile telephone frequency spectrum and thereby greatly increases the number of available control channels. The DCCH transmits control information to mobile stations along with information about neighboring cells (NCELLS).
The DCCH performs the function of a service access point for mobile subscribers to cells within a cellular telecommunications network. Thus, if a DCCH fails, the services provided by the cell are lost, resulting in decreased revenues for the network operator.
In cellular telecommunication networks, mobile calls must be processed on a continuous basis. Therefore, it is critical to have base stations operating on a continuous basis. If a DCCH is implemented on a base station's transmitter, and that transmitter subsequently fails, then all mobile stations within the base station's cell are no longer able to establish new communications with the base station. Therefore, it is desirable for base stations to have a backup control channel to ensure that a high service level is maintained in the associated cell.
Currently, the cellular industry uses a hardware solution to provide a backup for a failed primary DCCH. The hardware solution may be, for example, a conventional configuration of frequency agile transmitter combiners, hereinafter referred to as an auto-tuner. Typically, the auto-tuner is physically connected to the failed transmitter on which the primary DCCH was employed. Once the failure of the transmitter is detected, all DCCH operations within the transmitter are physically transferred to the auto-tuner via a hardware switch.
There are several disadvantages in using an auto-tuner for performing the DCCH backup function. First, each base station desiring a backup for its DCCH must be physically retro-fitted with an auto-tuner. Second, any changes or revisions to the logic for the auto-tuner requires changing the internal components of the hardware for each such auto-tuner. Finally, any changes or revisions to the hardware of an auto-tuner often requires service personal to physically visit each base station having an auto-tuner in order to make the revisions or changes. This is an expensive and wasteful solution.
It would be a distinct advantage to have a backup DCCH which overcomes these disadvantages. The present invention provides such a solution.